None.
Not Applicable.
This invention relates to steam turbines; and more particularly, to upstream wheel space pressure relief in a turbine so to eliminate steam balance holes now required in turbine blades to divert steam flow in certain conditions to maintaining acceptable thrust bearing loads.
Current steam turbine designs incorporate steam balance holes which are formed in the portion of a blade (bucket) where it attaches to a rotor, or in rotor disks. These holes provide a flow path for steam leakage through the seals of stationary turbine parts, through seals installed adjacent a rotor, and flow from the root of a stage between a nozzle and a bucket. Regardless of where the balance holes are located, their function is to maximize efficiency of turbine stage and keep thrust bearing loads within acceptable limits.
Over time, the size of balance holes has gotten progressively smaller. Recent analyses have shown that optimization (elimination) of the steam balance hole area results in significant improvement in the stage efficiency. Two reasons for this improvement are first, increases in energy reaction within a turbine stage, and second, an improved understanding of what actually happens within the stage. As these trends continue, it is reasonable to expect steam balance holes may eventually be eliminated altogether.
There are concerns, however. A primary concern is what will happen in emergency situations if there are no steam balance holes. This concern is particularly important in situations where a turbine stage is in its fully closed position. When this situation occurs, upstream wheel space pressure within the stage will approach that of the stage bowl, and the resulting high pressure will greatly increase the rotor""s axial thrust which can damage the rotor""s bearing necessitating turbine down time and expensive repairs. As is known in the art, axial thrust is normally balanced by a combination of balance pistons, steam flow paths running in opposite directions, split or double flow steam paths, and thrust bearings which are appropriately sized.
Large thrusts bearings have thermodynamic losses associated with oil flow, friction, and windage. These factors must be considered in the design of a turbine, and steam balance holes have heretofore been one way of doing so. If balance holes are eliminated, the effect of these factors must otherwise be accounted for. Further, in retrofit applications, the complexity and expense of modifying or replacing existing thrust bearings makes elimination of balance holes impractical.
The present invention provides a method by which steam balance holes can be eliminated while not effecting the desired thrust design.
Briefly stated, the present invention provides a modification to current design of a steam turbine by which steam balance holes are eliminated without effecting the performance of the turbine and which does not allow a rotor""s axial thrust to significantly increase even in emergency situations such as when the turbine stage is in its fully closed position. This arrangement will prevent thrust bearing failure. The modification comprises forming a plurality of grooves in the root setback face of a diaphragm for the stage. The grooves now provide a steam flow path between the diaphragm and a rotor/bucket assembly rather than the steam balance holes which heretofore have provided the path. The resultant flow path relieves upstream wheel space pressure even when the stage is fully closed. The steam balance holes can be eliminated.
The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. The invention allows elimination of the steam balance hole area, facilitating optimization of stage efficiency, whilst maintaining acceptable thrust design.